Chapter 7: Precipitation Processes

From: Introduction to Tropical Meteorology, 1st Edition, Version 1. 1 From: Introduction to Tropical Meteorology, 1st Edition, Version 1.1.2, Produced by the COMET® Program Copyright 2007-2008, University Corporation for Atmospheric Research.

From: Introduction to Tropical Meteorology, 1st Edition, Version 1. 1 From: Introduction to Tropical Meteorology, 1st Edition, Version 1.1.2, Produced by the COMET® Program Copyright 2007-2008, University Corporation for Atmospheric Research.

Growth of Cloud Droplet Forms of Precipitations Cloud Seeding

Precipitations Water Vapor Saturated Need Cloud Condensation Nuclei Cloud Droplet formed around Cloud Condensation Nuclei Need to fall down Precipitation

Radius = 100 times Volume = 1 million times

Terminal Velocity drag force Terminal velocity is the constant speed that a falling object has when the gravity force and the drag force applied on the subject reach a balance. Terminal velocity depends on the size of the object: small objects fall slowly and large objectives fall quickly. r V gravity force

Raindrops Rain droplets have to have large enough falling speed in order to overcome the updraft (that produces the rain) to fall to the ground. This means the rain droplets have to GROW to large enough sizes to become precipitation.

How Raindrop Grows? Growth by Condensation (too small) Growth in Warm Clouds: Collision-Coalescence Process Growth in Cool and Cold Clouds: Bergeron Process

Growth by Condensation Forms most cloud drops initially Growth limited to a radii of ~ 20 μm Insufficient for precipitation

Growth in Warm Clouds Most clouds formed in the Tropics, and many in the middle latitudes, are warm clouds (greater than 0ºC). The Collision-coalescence process generates precipitation. This process depends on the differing fall speeds of different-sized droplets. It begins with large collector drops which have high terminal velocities.

Collision Collector drops collide with smaller drops. Due to compressed air beneath falling drop, there is an inverse relationship between collector drop size and collision efficiency. Collisions typically occur between a collector and fairly large cloud drops. Smaller drops are pushed aside. Collision is more effective for the droplets that are not very much smaller than the collect droplet.

Coalescence When collisions occur, drops either bounce apart or coalesce into one larger drop. Coalescence efficiency is very high indicating that most collisions result in coalescence. Collision and coalescence together form the primary mechanism for precipitation in the tropics, where warm clouds dominate.

Cool and Cold Clouds A portion of most mid-latitude clouds have temperatures below the melting point of ice. Cold clouds are referred to those have temperature below 0ºC throughout and consist entirely of ice crystals, supercooled droplets, or a mixture of two. Cool clouds are referred to those have temperatures above 0ºC in the lower reaches and subfreezing condition above.

An Example of Cool and Cold Cloud Cumulonimbus clouds contain both ice (top, fuzzy cloud margins), liquid drops (bottom, sharp margins) and a mix of ice and liquid (middle)

Growth in Cool and Cold Clouds Cool month mid-latitude and high latitude clouds are classified as cool clouds as average temperatures are usually below freezing. Clouds may be composed of (1) Liquid water, (2) Supercooled water, and/or (3) Ice. Coexistence of ice and supercooled water is critical to the creation of cool cloud precipitation - the Bergeron Process.

Bergeron Process Saturation vapor pressure of ice is less than that of supercooled water and water vapor. Saturation vapor pressure gradient allows rapid growth of ice crystals

Riming and Aggregation Collisions between falling crystals and drops causes growth through riming and aggregation. Riming = liquid water freezing onto ice crystals producing rapid growth. Aggregation = the joining of multiple ice crystals through the bonding of surface water builds ice crystals to the point of overcoming updrafts Collision combined with riming and aggregation allow formation of precipitation within 1/2 hour of initial formation.

Forms of Precipitation Snow Rain Graupel and Hail Sleet Freezing Rain

Snow Snowflakes have a wide assortment of shapes and sizes depending on moisture content and temperature of the air. Snowfall distribution in North America is related to north-south alignment of mountain ranges and the presence of the Great Lakes.

Rain Rainshowers: are episodic precipitation events associated with convective activity and cumulus clouds Raindrop shape: As frictional drag increases, changes to a mushroom shape Drops eventually flatten Drops split when frictional drag overcomes the surface tension of water Splitting ensures a maximum drop size of about 5 mm and the continuation of the collision-coalescence process

Graupel and Hail Graupel are ice crystals that undergo extensive riming Lose six sided shape and smooth out Either falls to the ground or provides a nucleus for hail Hail are concentric layers of ice around graupel Up and downdraft interactions in thunderstorms Great Plains = highest frequency of hail events

Hail Formation

Concentric layers of ice in hail indicate the cyclical hailstone formation process

Hail Frequency in the U.S.

Sleet and Freezing Rain Sleet begins as ice crystals which melt into rain through a mid-level inversion before solidifying in colder near surface air Freezing Rain forms similarly to sleet, but does not fully refreeze

Twomey effect, 1974 How CCN (cloud condensation nuclei) from anthropogenic pollution may increase the cloud lifetime and decrease the precipitation

Cloud Seeding The objective is to convert some of the supercooled droplets in a cool clouds to ice and cause precipitation by the Bergeron process. Two primary methods are used to trigger the precipitation process. Dry ice is used to lower cloud temperature to a freezing point in order to stimulate ice crystal production leading to the Bergeron process. Silver iodide initiates the Bergeron process by directly acting as freezing nuclei. Under ideal conditions, seeding may enhance precipitation by about 10%.

Measuring Precipitation Standard raingages Sparse network provide a record of precipitation amount/time of the event

Raingage Measurement Errors Point estimates Wide variations across small spaces Overestimates Wind, residual water Precipitation Measurement by Weather Radar Precipitation estimates Real-time information

Doppler Radar Precipitation Estimates

Measuring Snow Accumulated snow measured Water equivalent of snow = 10 to 1 ratio Automated snow pillows Convert weight to water equivalent